An existing accessory dwelling unit ordinance governing the creation of an accessory dwelling unit by a local agency or an accessory dwelling ordinance adopted by a local agency shall provide an approval process that includes only ministerial provisions for the approval of accessory dwelling units and shall not include any discretionary processes, provisions, or requirements for those units, except as otherwise provided in this article. If a local agency has an existing accessory dwelling unit ordinance that fails to meet the requirements of this article, that ordinance shall be null and void and that agency shall thereafter apply the standards established in this article for the approval of accessory dwelling units, unless and until the agency adopts an ordinance that complies with this

article.